In the electrophotographic method, an electric latent image is generally formed by various techniques using a photoconductive substance, e.g., selenium, as a photoreceptor and the latent image is developed by attaching toners thereto, utilizing techniques such as the magnetic brush developing method.
In this developing step, a two-component type developer is most commonly used, in which carrier particles called "carriers" are used in combination with the toners in order to provide a suitable amount of positive or negative electricity. Various types of such carriers have been developed and put into practical use.
The carrier is required to have various characteristics. Particularly important among these characteristics are charging properties, impact resistance, abrasion resistance, developing properties, developer life, environmental independency in charge properties, and stability of charge properties in the passage of time.
In view of the above characteristics, conventional carriers remain inadequate; in fact, no sufficiently satisfactory carrier has yet been produced. For example, electrically conductive carriers, e.g., iron oxide powder, have the disadvantages that fine line reproducibility is poor, although solid developability is excellent, and further that it is necessary for a special charge controlling agent to be added to the toner in order to prolong the service life of the developer. Coated insulating carriers have the disadvantage that solid reproducibility is poor, although service life and reproducibility of fine lines are excellent. In order to overcome the above problems, small particle diameter carriers for magnetic brush development, containing a binder resin and fine magnetic particles dispersed therein, i.e., "microtoning carriers", have been proposed and put into practical use, as disclosed, for example, in U.S. Pat. No. 4,345,014 and Japanese Patent Application (OPI) No. 66134/79. (The term "OPI" as used herein means "unexamined published Japanese patent application.") These carriers, however, still present problems such as adhesion of carriers to a photoreceptor due to their small diameter in size, changes in charging properties under high and low humidity conditions which is caused by magnetic particles freed on the surface, and the further difficulty that since it is difficult to apply surface treatment, it is impossible to dramatically prolong the service life by coating.
It has now been discovered that the above problems of carriers containing fine magnetic particles in the dispersion state are basically caused by the methods of production conventionally used for the carriers.
Heretofore such magnetic powder-containing carriers have been produced either by (1) a method in which a mixture of a binder resin and a magnetic powder is melt kneaded, solidified and then pulverized to obtain carriers having the desired particle diameter, or (2) a method in which a magnetic powder is dispersed in a solution of a binder resin in a solvent, and the resulting dispersion is sprayed to evaporate the solvent at a high temperature (to about 150.degree. C.), thereby producing spherical carriers. Using the first method, it is difficult to produce spherical particles; the magnetic powder is readily freed because of surface irregularity; and it is difficult to attain particle diameters within the desired range. In the second method, since solidification proceeds with evaporation of the solvent, the resulting particle surface is porous and brittle, and coating treatment cannot be applied to the particles obtained.